Front-to-back bonding with through-substrate via (tsv)

ABSTRACT

Methods for forming a semiconductor device structure are provided. The method includes bonding a first wafer and a second wafer, and a first transistor is formed in a front-side of the first semiconductor wafer. The method further includes thinning a front-side of the second wafer and forming a second transistor in the front-side of the second wafer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending and commonly assigned patent applications: U.S. application Ser. No. 13/943,224, filed on Jul. 16, 2013 and entitled “Hybrid bonding with through substrate via (TSV)”, and U.S. application Ser. No. 13/943,245, filed on Jul. 16, 2013 and entitled “Mechanisms for forming three-dimensional integrated circuit (3DIC) stacking structure”, and U.S. application Ser. No. 13/943,401, filed on Jul. 16, 2013 and entitled “Hybrid bonding with through substrate via (TSV)”, and U.S. application Ser. No. 14/488,017, filed on Sep. 16, 2014 and entitled “Hybrid bonding with through substrate via (TSV), and U.S. application Ser. No. 14/752,342, filed on Jun. 26, 2015 and entitled “Method for forming hybrid bonding with through substrate via (TSV)”, and U.S. application Ser. No. 15/705,894, filed on Sep. 15, 2017 and entitled “Hybrid bonding with through substrate via (TSV) and entitled “Method for forming Hybrid bonding with through substrate via (TSV)”, the entire of which is incorporated by reference herein.

This application is a Continuation application of U.S. patent application Ser. No. 15/076,141, filed on Mar. 21, 2016, which is a Divisional application of U.S. patent application Ser. No. 13/943,157, filed on Jul. 16, 2013, the entire contents of which is incorporated by reference herein.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon. Many integrated circuits are typically manufactured on a single semiconductor wafer, and individual dies on the wafer are singulated by sawing between the integrated circuits along a scribe line. The individual dies are typically packaged separately, in multi-chip modules, or in other types of packaging, for example.

The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area. These smaller electronic components also require smaller packages that utilize less area than packages of the past, in some applications.

Three dimensional integrated circuits (3DICs) are a recent development in semiconductor packaging in which multiple semiconductor dies are stacked upon one another, such as package-on-package (PoP) and system-in-package (SiP) packaging techniques. Some 3DICs are prepared by placing dies over dies on a semiconductor wafer level. 3DICs provide improved integration density and other advantages, such as faster speeds and higher bandwidth, because of the decreased length of interconnects between the stacked dies, as examples. However, there are many challenges related to 3DICs.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1A-1F show cross-sectional representations of various stages of forming a semiconductor device, in accordance with some embodiments of the disclosure.

FIGS. 2A-2C show cross-sectional representations of various stages of forming a semiconductor device, in accordance with some embodiments of the disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Moreover, the performance of a first process before a second process in the description that follows may include embodiments in which the second process is performed immediately after the first process, and may also include embodiments in which additional processes may be performed between the first and second processes. Various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. Furthermore, the formation of a first feature over or on a second feature in the description may include embodiments in which the first and second features are formed in direct or indirectly contact.

Embodiments of the disclosure provide mechanisms of forming a semiconductor device. FIGS. 1A-1F show cross-sectional representations of various stages of forming a semiconductor device in accordance with some embodiments of the disclosure. Referring to FIG. 1A, a cross-sectional representation of a portion of a semiconductor wafer 100 and a portion of another semiconductor wafer 200 are shown in accordance with some embodiments.

Semiconductor wafer 100 includes a semiconductor substrate 104, which is made of silicon or other semiconductor materials and has a top surface 104 a and a bottom surface 104 b. Alternatively or additionally, semiconductor substrate 104 may include other elementary semiconductor materials such as germanium. In some embodiments, semiconductor substrate 104 is made of a compound semiconductor, such as silicon carbide, gallium arsenic, indium arsenide, or indium phosphide. In some embodiments, semiconductor substrate 104 is made of an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. In some embodiments, semiconductor substrate 104 includes an epitaxial layer. For example, semiconductor substrate 104 has an epitaxial layer overlying a bulk semiconductor.

Referring to FIG. 1A, device regions 103 are formed in a front-side 100 a of semiconductor wafer 100 in a front-end-of-line (FEOL) process in some embodiments. Each device regions 103 includes a gate structure 109 embedded in a dielectric layer 107, source/drain regions 110, and isolation structures 112, such as shallow trench isolation (STI) structures. Gate structure 109 includes a gate dielectric layer 106, a gate electrode 108, and spacers (not shown). Device regions 103 shown in FIG. 1 are merely an example, and other devices may be formed in device regions 103.

Device regions 103 may form various N-type metal-oxide semiconductor (NMOS) and/or P-type metal-oxide semiconductor (PMOS) devices, such as transistors or memories, and the like, interconnected to perform one or more functions. Other devices, such as capacitors, resistors, diodes, photo-diodes, fuses, and the like may also be formed in substrate 104. The functions of the devices may include memory, processing, sensors, amplifiers, power distribution, input/output circuitry, or the like. In some embodiments, device regions 103 include N-type metal-oxide semiconductor (NMOS) and/or P-type metal-oxide semiconductor (PMOS) transistors.

A metallization structure 122 is formed over substrate 104, e.g., over device regions 103. In some embodiments, metallization structure 122 includes interconnect structure, such as a contact plug 114 and conductive features 124. Conductive features 124 are embedded in an insulating material 126. Metallization structure 122 is formed in a back-end-of-line (BEOL) process in some embodiments. In some embodiments, contact plug 114 is made of conductive materials, such as copper, copper alloy, aluminum, alloys or combinations thereof. Conductive features 124 are also made of conductive materials. Alternatively, other applicable materials may be used. In some embodiments, contact plug 114 and conductive features 124 are made of conductive materials which are heat resistant, such as tungsten (W), Cu, Al, or AlCu. In some embodiments, insulating material 126 is made of silicon oxide. In some embodiments, insulating material 126 includes multiple dielectric layers of dielectric materials. One or more of the multiple dielectric layers are made of low dielectric constant (low-k) materials. In some embodiments, a top dielectric layer of the multiple dielectric layers is made of SiO₂. Metallization structure 122 shown is merely for illustrative purposes. Metallization structure 122 may include other configurations and may include one or more conductive lines and via layers.

A bonding layer 142, which is a dielectric layer, is formed over the front-side 100 a of semiconductor wafer 100, e.g. on metallization structure 122. In some embodiments, bonding layer 142 is made of a silicon-containing dielectric, such as silicon oxide, silicon oxynitride or silane oxide.

In some embodiments, bonding layer 142 is formed by plasma enhanced chemical vapor deposition (PECVD). In some other embodiments, bonding layer 142 is formed by a spin-on method. In some embodiments, bonding layer 142 has a thickness in a range from about 5 nm to about 300 nm.

As shown in FIG. 1A, devices, such as transistor including gate structure 109, are formed in front-side 100 a of semiconductor wafer 100, while no devices are formed in a backside 100 b of semiconductor wafer 100.

Semiconductor wafer 200 includes a substrate 204, which is similar to substrate 104. Substrate 204 has a top surface 204 a and a bottom surface 204 b. A bonding layer 242, which is a dielectric layer, is formed over a backside 200 b of semiconductor wafer 200, e.g. on bottom surface 204 b of substrate 204. In some embodiments, bonding layer 242 is similar to bonding layer 142. No devices are pre-formed in semiconductor wafer 200.

As shown in FIG. 1A, substrate 204 has a height H₁ from top surface 204 a to bottom surface 204 b in a range from about 50 μm to about 775 μm.

Before semiconductor wafers 100 and 200 are bonded together, two bonding layers 142 and 242 are treated. Bonding layers 142 and 242 are treated by a dry treatment or a wet treatment. The dry treatment includes a plasma treatment. The plasma treatment is performed in an inert environment, such as an environment filled with inert gas including N₂, Ar, He or combinations thereof. Alternatively, other types of treatments may be used. In some embodiments, both of bonding layers 142 and 242 are made of silicon oxide, and a plasma process is performed to bonding layers 142 and 242 to form Si—OH bonds on the surface of bonding layers 142 and 242 prior to bonding.

Referring to FIG. 1B, after the alignment is performed, semiconductor wafer 100 is boned to semiconductor wafer 200 to form a 3DIC stacking structure 300. As shown in FIG. 1B, when semiconductor wafer 200 is bonded to semiconductor wafer 100, bottom surface 204 b of substrate 204 faces top surface 104 a of substrate 104, and bonding layer 142 is bonded to bonding layer 242. Bonding of semiconductor wafers 100 and 200 is performed under pressure and heat. In some embodiments, the pressure for bonding is in a range from about 0.7 bar to about 10 bar. In some embodiments, the heat applied to semiconductor wafers 100 and 200 includes an anneal operation at a temperature in a range from about 20° C. to about 1000° C. The bonding process may be performed in an N₂ environment, an Ar environment, an He environment, an inert-mixing gas environment, or combinations thereof.

As shown in FIG. 1B, stacking structure 300 includes a bonding structure 150. Bonding structure 150 includes bonding layers 142 and 242 bonded together. Therefore, backside 200 b of semiconductor wafer 200 is bonded to front-side 100 a of semiconductor wafer 100. If some devices are formed in semiconductor wafer 200 before bonding, semiconductor wafers 100 and 200 have to be precisely aligned before bonding. In contrast, since no devices are pre-formed in semiconductor wafer 200, an aligning process is not required for bonding of semiconductor wafers 100 and 200.

After bonding semiconductor wafers 100 and 200, a thinning process 11 is performed on top surface 204 a of semiconductor wafer 200, referring to FIG. 1C. Thinning process 11 may include a grinding operation and a polishing operation (such as chemical mechanical polishing (CMP)). After thinning process 11, a wet etching operation is performed to remove the defects formed on top surface 204 a of semiconductor wafer 200. After thinning process 11, substrate 204 has a height H₂ from top surface 204 a′ to bottom surface 204 b of semiconductor substrate 204 in a range from about 0.2 μm to about 10 μm. Height H₂ is smaller than height H₁. In some embodiments, and a ratio of height H₂ to H₁ is in a range from about 0.0002 to about 0.99.

After thinning semiconductor wafer 200, device regions 203 are formed in front-side 200 a of semiconductor wafer 200, referring to FIG. 1D. Device regions 203 are formed in a front-end-of-line (FEOL) process in some embodiments. Each device regions 203 includes a gate structure 209 embedded in a dielectric layer 207, source/drain regions 210, and isolation structures 212, such as shallow trench isolation (STI) structures. Gate structure 209 includes a gate dielectric layer 206, a gate electrode 208, and spacers (not shown). A contact plug 214 is formed over device regions 203, e.g. on gate structure 209. In some embodiments, formations of device regions 203 and contact plug 214 are performed under a relatively low temperature, such that metallization structure 122 will not be damaged during the processes. However, in some embodiments, metallization structure 122, including contact plug 114 and conductive features 124, are made of heat resistant conductive materials, and thus formations and materials of device regions 203 and contact plug 214 can be similar to those of device regions 103 and contact plug 114. In some embodiments, device regions 203 include NMOS and/or PMOS transistors.

As shown in FIG. 1D, devices, such as transistors including gate structure 209, are formed in front-side 200 a of semiconductor wafer 200, while no devices are formed in backside 200 b of semiconductor wafer 200. In addition, backside 200 b of semiconductor wafer 200 is bonded to front side 100 a of semiconductor wafer 100, and therefore the resulting stacking structure 300 is a front-to-back (face-to-back) stacking structure.

After device regions 203 are formed, through-substrate via (TSV) 400 is formed through second semiconductor wafer 200, referring to FIG. 1E. Through-substrate via (TSV) 400 is used to provide electrical connections and for heat dissipation for 3DIC stacking structure 300. Although FIG. 1E only shows one TSV, more than one TSV may be formed to pass through second semiconductor wafer 200.

TSV 400 includes a liner 410, a diffusion barrier layer 420, and a conductive material 430. TSV 400 is formed by the following operations. Firstly, a TSV opening is formed extending to conductive feature 124 a of semiconductor wafer 100 by one or more etching processes. After the TSV opening is formed, liner 410 is formed on sidewalls of the TSV opening to act as an isolation layer, such that conductive materials of TSV 400 and semiconductor substrate 204 do not directly contact with each other. Afterwards, diffusion barrier layer 420 is conformally formed on liner 410 and on the bottom of the TSV opening. Diffusion barrier layer 420 is used to prevent conductive material 430, which will be formed later, from migrating to device regions 103 and 203. After diffusion barrier layer 420 is formed, conductive material 430 is used to fill into the TSV opening. Afterwards, excess liner 410, diffusion barrier layer 420, and conductive material 430, which are on the outside of the TSV opening, are removed by a planarization process, such as a chemical mechanical polishing (CMP) process, although any suitable removal process may be used.

Liner 410 is made of an insulating material, such as oxides or nitrides. Liner 410 may be formed by using a plasma enhanced chemical vapor deposition (PECVD) process or other applicable processes. Liner 410 may be a single layer or multi-layers. In some embodiments, liner 410 has a thickness in a range from about 100 Å to about 5000 Å.

In some embodiments, diffusion barrier layer 420 is made of Ta, TaN, Ti, TiN or CoW. In some embodiments, diffusion barrier layer 420 is formed by a physically vapor deposition (PVD) process. In some embodiments, diffusion barrier layer 420 is formed by plating. In some embodiments, conductive material 430 is made of copper, copper alloy, aluminum, aluminum alloys, or combinations thereof. Alternatively, other applicable materials may be used.

As shown in FIG. 1E, in some embodiments, TSV 400 has a width W₁ in a range from 0.025 μm to about 4 μm. In some embodiments, TSV 400 has a depth D₁ in a range from 0.2 μm to about 10 μM. In some embodiments, TSV 400 has an aspect ratio (D₁/W₁) in a range from 2 to about 15.

As shown in FIG. 1E, TSV 400 is used to connect conductive features 124 a on semiconductor wafer 100 to second semiconductor wafer 200. If a TSV similar to TSV 400 is formed in semiconductor wafer 200 with a height H₁ as shown in FIG. 1B, the TSV will have a higher aspect ratio than TSV 400. With high aspect ratio, filling materials into the TSV opening becomes challenging. Voids may form in the TSV opening. In addition, due to insufficient sidewall coverage of liner 410 or diffusion barrier layer 420, some extrusion or diffusion problems related to conductive via material 430 may occur. In contrast, as shown in FIG. 1E, since the height of semiconductor wafer 200 is reduced from H₁ to H₂, TSV 400 has a relative smaller aspect ratio. Therefore, the void problems and the extrusion or diffusion problems resulting from a high aspect ratio of the TSV are resolved or greatly reduced. In addition, the overall package height of 3DIC stacking structure 300 is reduced to meet advanced packaging requirements. Therefore, 3DIC stacking structure 300 achieves small form factor.

In addition, devices in the vicinity of the TSV suffer from serious performance degradation due to the stress induced by the TSV. A keep-out zone (KOZ) is used to define a region where no devices could be placed within. In some embodiments, keep-out zone (KOZ) is defined by a distance W₂, which is measured from a sidewall 400 a of TSV 400 to a nearest gate structure 209. Since semiconductor wafer 200 has a relatively small height H₂ due to thinning, the depth D₁ of TSV 400 is made smaller, resulting in a smaller width W₁. Therefore, overall stress induced by TSV 400 is reduced, and distance W₂ is also made smaller in FIG. 1E. In some embodiments, distance W₂ is in a range from about 0.01 μm to about 3 μm. When distance W₂ is smaller, a larger area of device regions 203 may be used. As a result, integrated intensity of devices in device regions 203 is further improved.

After TSV 400 is formed, an interconnect structure 500 is formed on front-side 200 a of second semiconductor wafer 200, referring to FIG. 1F. Interconnect structure 500 is electrically connected to conductive features 124 a of semiconductor wafer 100 via TSV 400. Interconnect structure 500 includes conductive features 510, such as conductive lines, vias, or conductive pads, formed in an insulating material 520. The metal routings of the conductive features shown in FIG. 1F, are merely examples. Alternatively, other designs of metal routings of conductive features may be used according to actual application.

As shown in FIG. 1F, in stacking structure 300, devices, such as transistors inducing gate structure 109, are formed in front-side 100 a of semiconductor wafer 100 while no devices are formed in backside 100 b of semiconductor wafer 100. Devices, such as transistors including gate structure 209, are formed in front-side 200 a of semiconductor wafer 200 while no devices are formed in backside 200 b of semiconductor wafer 200. Backside 200 b of semiconductor wafer 200 is bonded to front-side 100 a of semiconductor wafer 100, and therefore stacking structure 300 is a front-to-back structure. In addition, interconnect structure 500 is formed on front-side 200 a of semiconductor wafer 200, and TSV 400 directly contacts interconnect structure 500 and conductive feature 124 a of semiconductor wafer 100.

If devices are pre-formed on semiconductor wafer 200 before bonding to semiconductor wafer 100, semiconductor wafer 200 cannot be thinned since the devices are located on front-side 200 a of semiconductor wafer 200. Therefore, it will be difficult to form a small TSV. In contrast, no devices are pre-formed on semiconductor wafer 200, and therefore semiconductor wafer 200 can be thinned from top surface 204 a of substrate 204. After thinning process 11, the devices, such as transistors including gate structure 209, and TSV 400 could be subsequently formed in front-side 200 a of semiconductor wafer 200. Therefore, a relatively small TSV 400 is formed in front-to-back 3DIC stacking structure 300.

In addition, other processes may also be performed to 3DIC stacking structure 300, and 3DIC stacking structure 300 may be diced to form individual chips afterwards.

FIGS. 2A-2C show cross-sectional representations of various stages of forming a semiconductor device, in accordance with some embodiments of the disclosure. As shown in FIG. 2A, the 3D stacking structure 300′ in FIG. 2A is almost the same with 3D stacking structure 300 except an additional TSV 400 b is formed in semiconductor wafer 200. The formation of stacking structure 300′ may involve additional patterning and etching processes. For example, a first TSV opening is formed through semiconductor wafer 200 to expose a portion of a conductive feature 124 a of semiconductor wafer 100. After the first TSV opening is formed, a second TSV opening is formed through semiconductor wafer 200 to extend over device regions 103 of semiconductor wafer 100. Afterwards, liners 410 b and 410, diffusion barrier layers 420 b and 420, and conductive materials 430 b and 430 are used to fill in the first and second TSV openings to form TSVs 400 b and 400. As shown in FIG. 2A, TSV 400 b includes liner 410 b similar to liner 410, diffusion barrier layer 420 b similar to diffusion layer 420, and conductive material 430 b similar to conductive material 430, but TSV 400 b further extends to a region in backside 100 b of semiconductor wafer 100.

In some embodiments, TSV 400 b has a width W₃ in a range from about 0.3 μm to about 10 μm. In some embodiments, TSV 400 b has a depth D₂ in a range from about 15 μm to about 100 μm. In some embodiments, TSV 400 b has an aspect ratio (D₂/W₃) in a range from about 5 to about 15. A ratio of depth D₂ to depth D₁ is in a range from about 2 to about 15.

Referring to FIG. 2B, interconnect structure 500 is formed on front-side 200 a of semiconductor wafer 200. Interconnect structure 500 includes conductive features, such as conductive lines, vias, or conductive pads, formed in an insulating material. The metal routings of the conductive features shown in FIG. 2B, are merely an example. Alternatively, other designs of metal routings of conductive features may be used according to actual application. In some embodiments, substrate 104 has a height H₃ in a range from about 10 μm to about 100 μm.

After forming interconnect structure 500, semiconductor wafer 100 is thinned from bottom surface 104 b of substrate 104 to expose a bottom of TSV 400 b, referring to FIG. 2C. After the thinning process, redistribution (RDL) structure 160 is formed on backside 100 b of semiconductor wafer 100 (e.g. thinned bottom surface 104 b″ of substrate 104). RDL structure 160 includes a metal pad 162 formed in a passivation layer 164. Metal pad 162 is electrically connected to exposed TSV 400 b. In some embodiments, metal pad 162 is made of conductive materials with low resistivity, such as copper (Cu), aluminum (Al), Cu alloys, Al alloys, or other applicable materials. Although FIG. 2C only shows one RDL structure 160, more than one RDL structures may be formed.

An under bump metallization (UBM) layer 165 is formed on metal pad 162, and a conductive element 166 (such as solder ball) is formed over UBM layer 165. UBM layer 165 may contain an adhesion layer and/or a wetting layer. In some embodiments, UBM layer 165 is made of titanium (Ti), titanium nitride (TiN), tantalum nitride (TaN), tantalum (Ta), or the like. In some embodiments, UBM layer 165 further includes a copper seed layer. In some embodiments, conductive element 166 is made of conductive materials having low resistivity, such as solder or solder alloy. Exemplary elements included in the solder alloy may include Sn, Pb, Ag, Cu, Ni, Bi, or combinations thereof.

In some embodiments, interconnect structure 500 is electrically connected to another package (not shown) on the backside 100 b of semiconductor wafer 100 via TSV 400 b, RDL structure 160 and conductive element 166. In some embodiments, additional conductive structures, such as additional RDL structures, are formed over interconnect structure 500, such that semiconductor wafers 100 and 200 can be connected to other package substrates (not shown).

As shown in FIG. 2C, TSVs 400 and 400 b individually provide different functions. TSV 400 directly contacts interconnect structure 500 and conductive feature 124 a, and TSV 400 b directly contacts interconnect structure 500 and RDL structure 160. Therefore, semiconductor wafer 200 is electrically connected to semiconductor wafer 100 via TSV 400, and semiconductor wafer 200 is electrically connected to another package structure (not shown) via interconnect structure 500 and TSV 400 b. TSVs 400 and 400 b provide a fast conductive path to connect semiconductor wafer 100, semiconductor wafer 200, and/or other package structures, without the formation of complicated metal routings.

Embodiments of mechanisms for forming a semiconductor device are provided. A backside of a second semiconductor is bonded to a front-side of a first semiconductor, which has devices, such as transistors, formed therein. After the second semiconductor wafer is bonded to the first semiconductor wafer, the front-side of the second semiconductor wafer without devices pre-formed therein is thinned. After the thinning process, devices, such as transistors, are formed in the front-side of the second semiconductor wafer to form a front-to-back (face-to-back) stacking structure. A relatively small TSV is formed in the front-to-back stacking structure. Alternatively, TSVs having different sizes are formed in the front-to-back stacking structure.

In some embodiments, a method for forming a semiconductor device structure is provided. The method includes bonding a first wafer and a second wafer, and a first transistor is formed in a front-side of the first semiconductor wafer. The method further includes thinning a front-side of the second wafer and forming a second transistor in the front-side of the second wafer.

In some embodiments, a method for forming a semiconductor device structure is provided. The method includes bonding a first semiconductor wafer to a second semiconductor wafer by a bonding layer, and the bonding layer is formed between the first semiconductor wafer and the second semiconductor wafer. The method includes thinning a portion of the second semiconductor wafer to form an exposed surface and forming a second transistor in the exposed surface of the second semiconductor wafer. The method also includes forming a first TSV in the second semiconductor wafer, wherein the first TSV passes through the bonding layer.

In some embodiments, a method for forming a semiconductor device structure is provided. The method includes bonding a first semiconductor wafer and a second semiconductor wafer and thinning a portion of the second semiconductor wafer. The method also includes forming a second transistor in a front-side of the second semiconductor wafer and forming a first TSV through the second semiconductor wafer. The method further includes forming a second TSV through the second semiconductor wafer and the first semiconductor wafer.

Although embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present disclosure. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method for forming a semiconductor device structure, comprising: bonding a first wafer and a second wafer, wherein a first transistor is formed in a front-side of the first wafer; thinning a front-side of the second wafer; and forming a second transistor in the front-side of the second wafer.
 2. The method as claimed in claim 1, further comprising: forming at least one first TSV in the second wafer, wherein the first TSV directly contacts a conductive feature of the first wafer; and forming at least one second TSV in the first wafer and the second wafer to directly contact a redistribution (RDL) structure formed over a backside of the first wafer.
 3. The method as claimed in claim 2, further comprising: thinning a backside of the first wafer to expose the second TSV.
 4. The method as claimed in claim 2, wherein the second TSV has a second height greater than a first height of the first TSV.
 5. The method as claimed in claim 2, further comprising: forming a contact plug over the first transistor, wherein a top surface of the contact plug is level with a top surface of the first TSV.
 6. The method as claimed in claim 1, further comprising: forming an interconnect structure over the front-side of the second wafer, wherein the interconnect structure is electrically connected to the conductive feature of the first wafer.
 7. The method as claimed in claim 1, wherein the step of bonding the first wafer and the second wafer is performed by using a first bonding layer and a second bonding layer, and the first bonding layer and the second bonding layer made of silicon oxide, silicon oxynitride or silane oxide.
 8. The method as claimed in claim 1, wherein no devices are formed in the second wafer before the step of thinning the front-side of the second wafer.
 9. A method for forming a semiconductor device structure, comprising: bonding a first semiconductor wafer to a second semiconductor wafer by a bonding layer, wherein the bonding layer is formed between the first semiconductor wafer and the second semiconductor wafer; thinning a portion of the second semiconductor wafer to form an exposed surface; forming a second transistor in the exposed surface of the second semiconductor wafer; and forming a first TSV in the second semiconductor wafer, wherein the first TSV passes through the bonding layer.
 10. The method as claimed in claim 9, further comprising: forming an interconnect structure over the second semiconductor wafer, wherein the interconnect structure is electrically connected to a conductive feature of the first semiconductor wafer through the first TSV.
 11. The method as claimed in claim 10, wherein the interconnect structure comprises a first conductive line and a second conductive line, the first conductive line has a first surface facing a top surface of the first TSV.
 12. The method as claimed in claim 9, further comprising: forming a second TSV in the second semiconductor wafer and the first semiconductor wafer; and forming a first redistribution (RDL) structure on a backside of the first semiconductor wafer, wherein the second TSV contacts with the first RDL structure.
 13. The method as claimed in claim 12, wherein forming the first RDL structure comprises: forming a metal pad on a bottom surface of the second TSV; forming an under bump metallization (UBM) layer on the metal pad; and forming a conductive element on the UBM layer
 14. The method as claimed in claim 9, wherein a first transistor is formed over a front-side of the first semiconductor wafer, and a conductive feature is formed over the first transistor, and the first TSV contacts with the conductive feature.
 15. A method for forming a semiconductor device structure, comprising: bonding a first semiconductor wafer and a second semiconductor wafer; thinning a portion of the second semiconductor wafer; forming a second transistor in a front-side of the second semiconductor wafer; forming a first TSV through the second semiconductor wafer; and forming a second TSV through the second semiconductor wafer and the first semiconductor wafer.
 16. The method as claimed in claim 15, further comprising: thinning a backside of the first semiconductor wafer to expose the second TSV; and forming a first redistribution (RDL) structure on a backside of the first semiconductor wafer, wherein the second TSV directly contacts with the first RDL structure.
 17. The method as claimed in claim 15, wherein the step of bonding the first semiconductor wafer and the second semiconductor wafer is performed by using a first bonding layer and a second bonding layer, and the first bonding layer and the second bonding layer made of silicon oxide, silicon oxynitride or silane oxide.
 18. The method as claimed in claim 15, wherein the first TSV as a first depth, the second TSV has a second depth greater than the first depth.
 19. The method as claimed in claim 15, wherein a first transistor is formed over a front-side of the first semiconductor wafer, and a conductive feature is formed over the first transistor, and the first TSV contacts with the conductive feature.
 20. The method as claimed in claim 15, further comprising: forming an interconnect structure over the front-side of the second semiconductor wafer, wherein the interconnect structure comprises a first conductive line and a second conductive line, the first conductive line has a first surface facing a top surface of the first TSV. 